In parallel magnetic resonance imaging, a portion of the k-space being measured is acquired by multiple antenna elements simultaneously. The data acquired by the multiple antenna elements is combined to make a complete magnetic resonance image. Typically sensitive surface coil elements are placed around a subject. The image data collected by each of the multiple antenna elements is combined together using spatially dependent coil sensitivity for each coil element.
Typically the coil sensitivities are determined during a calibration step. One way of calibrating the multiple elements is to acquire data at the same time using a Quadrature Body Coil (QBC) as well as the multiple antenna elements. A QBC is not particularly sensitive; however they acquire data with a good spatial uniformity which can be used to construct a reference image. The coil sensitivity for a particular coil element can be calculated by dividing the image from the coil element by the reference image.
In the journal article Uccker et. al., “ESPIRiT—An Eigenvalue Approach to Autocalibrating Parallel MRI: Where SENSE Meets GRAPPA,” Magnetic Resonance in Medicine, online, May 6, 2013 discloses a method of calculating coil sensitivities by a k-space based method to calculate coil sensitivities as Eigen vectors of a linear system.
U.S. Pat. No. 6,380,741 B1 discloses using a pair of r.f. receive coils together with sensitive information concerning those coils to unfurl the aliased images to produce a full image.